Phosphatidylinositol 3-kinase (PI 3-kinase) is stimulated by insulin and a variety of growth factors, but its exact role in signal transduction remains unclear. We have used a novel, highly specific inhibitor of PT 3-kinase to dissect the role of this enzyme in insulin action. Treatment of intact 3T3-L1 adipocytes with LY294002 produced a dose-dependent inhibition of insulin-stimulated PI 3-kinase (50% inhibitory concentration, 6 microM) with > 95% reduction in the levels of phosphatidylinositol-3,4,5-trisphosphate without changes in the levels of phosphatidylinositol-4-monophosphate or its derivatives. In parallel, there was a complete inhibition of insulin-stimulated phosphorylation and activation of pp70 S6 kinase. Inhibition of PI 3-kinase also effectively blocked insulin- and serum-stimulated DNA synthesis and insulin-stimulated glucose uptake by inhibiting translocation of GLUT 4 glucose transporters to the plasma membrane. By contrast, LY294002 had no effect on insulin stimulation of mitogen-activated protein kinase or pp90 S6 kinase. Thus, activation of PI 3-kinase plays a critical role in mammalian cells and is required for activation of pp70 S6 kinase and DNA synthesis and certain forms of intracellular vesicular trafficking but not mitogen-activated protein kinase or pp90 S6 kinase activation. These data suggest that PI 3-kinase is not only an important component but also a point of divergence in the insulin signaling network.

In differentiated 3T3-F442A adipocytes, insulin stimulated rapid and transient phosphorylation of c-Jun. Insulin also stimulated phosphorylation of c-Fos and several Fos-related proteins (pp72, pp45, and pp39) as indicated by precipitation with anti-c-Fos antibody following exposure to denaturating conditions. Phosphorylation of c-Fos was stimulated by 7-fold by 60 min, while phosphorylation of Fos-related proteins reached maxima of 3.5-5.5-fold at 15 to 60 min. The increase in phosphorylated c-Fos was due to an increase in both c-Fos protein and the stoichiometry of c-Fos phosphorylation, and was not observed in c-fos (-/-) cells. Additionally, insulin stimulated phosphorylation of a protein with molecular mass of approximately 82 kDa on tyrosine residues by 2.5-fold within 30 min; this protein appeared to be immunologically related to c-Fos. These increases in the phosphorylation of AP-1 transcription factors correlated with a > 5-fold stimulation of expression of a 12-O-tetradecanoylphorbol-13-acetate-responsive element-chloramphenicol acetyltransferase reporter gene transiently transfected into 3T3-F442A cells. These results indicate that insulin stimulates the phosphorylation of AP-1 transcription factors and several Fos-related proteins on serine and tyrosine residues. This is associated with changes in AP-1-mediated gene expression in vivo, suggesting that AP-1 phosphorylation by insulin plays a role in insulin-regulated gene expression.

The mouse IRS-1 gene has been cloned and its structure determined. Mouse IRS-1 differs from rat by the absence of the potential C-terminal nucleotide binding site. Otherwise, the predicted IRS-1 protein is highly conserved between mouse, rat and humans, especially in the possible phosphorylation sites. The highly conserved nature of IRS-1 suggests the importance of these domains in the function of IRS-1 or its association with other proteins.

Dexamethasone treatment of IM-9 lymphocytes and Fao hepatoma cells resulted in an increase in synthesis of the insulin receptor. The receptors synthesized after stimulation with the glucocorticoid had altered carbohydrate structure. The carbohydrate side chains of the insulin receptor were less branched on the dexamethasone-treated cells; i.e., the ratio of saccharides with three and four branches to those bearing only two branches was decreased. The predominant polymannose oligosaccharide after dexamethasone treatment was Man9GlcNAc (vs Man6GlcNAc in the control cell). Both of these changes are consistent with a less complete processing of the N-linked carbohydrate units and were not observed for the total cellular glycoproteins, whereas all glycoproteins manifested an increased sialylation in Fao cells after dexamethasone treatment. These data indicate that glucocorticoid treatment results in alterations in branching of carbohydrate side chains, in the size of polymannose chains and in sialylation of the insulin receptor.

Insulin rapidly stimulates tyrosine kinase activity of its receptor resulting in phosphorylation of its cytosolic substrate insulin receptor substrate 1 (IRS-1), which in turn associates with and activates the enzyme phosphatidylinositol 3-kinase (PI 3-kinase). In the present study we have examined these three initial steps in insulin action during the differentiation of 3T3-F442A adipocytes and after treatment with dexamethasone or insulin. The differentiation of 3T3-F442A cells was characterized by a 13-fold increase in insulin receptor protein, a 9-fold increase in IRS-1, and a 10- and 4.5-fold increase in their insulin-stimulated phosphorylation, respectively. The mRNA expression of these two proteins showed a similar 8-fold increase during differentiation. In addition there was a 3.5-fold increase in PI 3-kinase protein [85 kilodalton (kDa) subunit] and a 16-fold increase in IRS-1-associated PI 3-kinase activity between day 0 and day 8 of differentiation. Dexamethasone (1 microM) treatment of differentiated cells induced a further 48% (P 0.05) increase in insulin receptor level, but the autophosphorylation of the receptor was decreased by 31 +/- 1% (P 0.02). At the same time there was a decrease by 56 +/- 4% (P 0.005) in IRS-1 protein and by 31 +/- 1% (P 0.001) in IRS-1 phosphorylation. The expression of insulin receptor mRNA was unchanged, but the expression of IRS-1 mRNA was decreased by approximately 75% after dexamethasone. By contrast, dexamethasone induced a 69% increase in the level of PI 3-kinase as determined by immunoblotting. The combined effect of decreased IRS-1 phosphorylation and increased PI 3-kinase protein was a minimal change (15% decrease) in the association/activation between IRS-1 and PI 3-kinase. Chronic treatment with 100 nM insulin induced a time- and dose-dependent decrease in insulin receptor and IRS-1 protein levels reaching a nadir of 34 +/- 5% (P 0.005) and 39 +/- 5% (P 0.01) of control levels after 24 h, respectively. There was an even more marked decrease in the phosphorylation level of these proteins. Chronic insulin treatment also produced a 30% decrease in PI 3-kinase protein levels and a approximately 50% decrease in the association/activation between IRS-1/PI 3-kinase. The expression of insulin receptor and IRS-1 mRNA was unchanged during chronic insulin treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of insulin receptor tyrosine kinase activity in stimulation of intracellular enzymes linked to insulin action [phosphatidylinositol 3-kinase (PtdIns 3-kinase), microtubule-associated protein (MAP) kinase, and S6 kinases] was studied in Chinese hamster ovary cells which overexpress wild type human insulin receptors, receptors with reduced kinase activity due to substitution of Phe for Tyr1146 (single-Phe), Tyr1150,1151 (double-Phe), and Tyr1146,1150,1151 (triple-Phe), or kinase-inactive receptors with a substitution of Ala for Lys1018 in the ATP binding site (A1018). We have previously shown that receptor autophosphorylation and kinase activity of these mutants were reduced by approximately 50, 65, 85, and 100%, respectively. Glycogen and DNA synthesis parallel the level of receptor autophosphorylation and kinase activity; however, receptor serine and threonine phosphorylation was independent of receptor tyrosine kinase activity and receptor internalization was completely dependent on maximal receptor kinase activity. Overexpression of the wild type insulin receptor increased both maximal insulin receptor substrate-1-associated and total insulin-stimulated PtdIns 3-kinase activity, as well as S6 and MAP kinase activities 2.0- to 3.6-fold. In addition there was a leftward shift of the dose-response curves for PtdIns 3-kinase and S6 kinases by approximately 10-fold. Expression of the single- and double-Phe mutant receptors also enhanced maximal PtdIns 3-kinase activity, but had no effect on insulin sensitivity, whereas expression of either the triple-Phe or kinase-inactive receptors did not enhance insulin stimulation or increase insulin sensitivity as compared to the control cells. When comparing the mutant and wild type receptors, differences in insulin sensitivity were least for insulin-stimulated MAP kinase and greatest for S6 kinase; with the latter there was greater than a 1000-fold difference in insulin sensitivity when cells that overexpress wild type vs. kinase-inactive insulin receptors were compared. Thus, the level of insulin receptor tyrosine autophosphorylation and kinase activity regulate both maximal activation and insulin sensitivity of these intracellular kinases in the insulin action pathway which may lead to glycogen and/or DNA synthesis. The differential sensitivity of these enzymes to changes in receptor activation suggests that they may be differently coupled to the receptor kinase.

The activation of p21ras by receptor tyrosine kinases involves the translocation of the growth factor receptor bound protein 2-mammalian son of sevenless protein (Grb2-SOS) complex to the plasma membrane where p21ras is localized. Insulin receptors induce p21ras-GTP formation by two possible mechanisms: tyrosine phosphorylation of insulin receptor substrate 1 (IRS1) and its subsequent association with Grb2, or Shc phosphorylation and its subsequent association with Grb2. We investigated the contribution of the major tyrosine autophosphorylation sites Tyr1158, Tyr1162, and Tyr1163 of the insulin receptor to IRS1.Grb2 and Shc.Grb2 association and the formation of p21ras-GTP. Chinese hamster ovary-derived cell lines were used overexpressing mutant insulin receptors in which the major tyrosine autophosphorylation sites were stepwise replaced by phenylalanines. In cell lines expressing wild type or mutant Y1158F,Y1162,Y1163 (FYY) receptors, insulin stimulated tyrosine phosphorylation of IRS1 and Shc and the formation of IRS1.Grb2 and Shc.Grb2 protein complexes, together with an increase in p21ras-GTP. Cell lines expressing mutant Y1158,Y1162F,Y1163F (YFF) receptors showed insulin-induced tyrosine phosphorylation of Shc, Shc.Grb2 complex formation, and p21ras-GTP formation, whereas tyrosine phosphorylation of IRS1 was strongly decreased and formation of IRS1.Grb2 complexes was undetectable. The activity of FYY and YFF receptors to mediate p21ras-GTP formation correlated with their activity to induce Shc phosphorylation and Shc.Grb2 association. The mutant insulin receptors Y1158F,Y1162F,Y1163 and Y1158F,Y1162F,Y1163F were inactive in inducing any of these responses. We conclude that phosphorylation of Tyr1158 and Tyr1162 of the insulin receptor is linked to distinct post-receptor processes and that YFF receptors generate p21ras-GTP via the Shc.Grb2 pathway rather than one involving IRS1.Grb2 interaction.